cmd/compile: better error message when generics are used with -lang=go1.17

[mdempsky]

Currently, when compiling with -G=3 -lang=go1.17, we run the types2 typechecker, but parse without the AllowGenerics flag. The result is when users try to write generic code, they'll get syntax error messages like unexpected [, expecting (, which seems to be a recurring source of confusion and leaves users (incl. expert Go users) uncertain of how to proceed.

I think we should probably instead always parse with AllowGenerics (at least when we're going to use types2), and have types2 report the usual allowVersion error messages if users try to use generics with go1.17 or earlier.

/cc @griesemer @findleyr

[griesemer]

Agreed. This makes sense now that we're on 1.18. I will take care of this (but I'm ooo until next Monday).

[gopherbot]

Change https://golang.org/cl/344871 mentions this issue: cmd/compile: always accept 1.18 syntax but complain if not 1.18